Go receiver implicit pointer conversion not working

Question

I'm learning Go, and I was following the go tour.

In the exercise about Stringer, here , Implementing the function with a *IPAddr receiver doesn't seem to work, which the go tour describes as should work.

package main

import "fmt"

type IPAddr [4]byte

// TODO: Add a "String() string" method to IPAddr.
func (ip *IPAddr) String() string {
    return fmt.Sprintf("%v.%v.%v.%v", ip[0], ip[1], ip[2], ip[3])
}
func main() {
    hosts := map[string]IPAddr{
        "loopback":  {127, 0, 0, 1},
        "googleDNS": {8, 8, 8, 8},
    }
    for name, ip := range hosts {
        fmt.Printf("%v: %v\n", name, ip)
    }
}

Output is:

loopback: [127 0 0 1]
googleDNS: [8 8 8 8]

But changing String() string to func (ip IPAddr) from func (ip *IPAddr) gives the correct output:

loopback: 127.0.0.1
googleDNS: 8.8.8.8

Why is that?

Answer 1

Implementing func (ip IPAddr) String() will work for both IPAddr and *IPAddr types.

Implementing func (ip *IPAddr) String will only work for *IPAddr .

Answer 2

The implicit conversion means that you can call the function on either a value, or pointer, but it does not satisfy the implementation of an interface. If you implement an interface with a pointer receiver, the pointer must be used in the function call.

The code below shows the *IPAddr used with the Stringer interface, and an IPAddr used with the new foo() function (also implemented with a pointer receiver):

package main

import "fmt"

type IPAddr [4]byte

// TODO: Add a "String() string" method to IPAddr.
func (ip *IPAddr) String() string {
    return fmt.Sprintf("%v.%v.%v.%v", ip[0], ip[1], ip[2], ip[3])
}

func (ip *IPAddr) foo() string {
    return "bar"
}

func main() {
    hosts := map[string]IPAddr{
        "loopback":  {127, 0, 0, 1},
        "googleDNS": {8, 8, 8, 8},
    }
    for name, ip := range hosts {
        fmt.Printf("%v: %v\n", name, &ip)

    fmt.Println(ip.foo())
    }
}